Ion implantation is a standard technique for introducing conductivity-altering impurities into a workpiece. A desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the workpiece. The energetic ions in the ion beam penetrate into the bulk of the workpiece material and are embedded into the crystalline lattice of the workpiece material to form a region of desired conductivity.
Solar cells are one example of a device that uses silicon workpieces. Any reduced cost to the production of high-performance solar cells or any efficiency improvement to high-performance solar cells would have a positive impact on the implementation of solar cells worldwide. Ion implantation is one such process that can reduce the cost while improving the efficiency of solar cells. This will enable the wider availability of this clean energy technology.
Solar cells typically consist of a p-n semiconducting junction. FIG. 1 is a cross-sectional view of an interdigitated back contact (IBC) solar cell. In the IBC solar cell 205, the junction is on the back or non-illuminated surface. In this particular embodiment, the IBC solar cell 205 has an n-type base 206, an n+ front surface field 207, a passivating layer 208, and an anti-reflective coating (ARC) 209. The passivating layer 208 may be SiO2 in one instance, though other dielectrics may be used. Photons 214 enter the IBC solar cell 205 through the top (or illuminated) surface, as signified by the arrows. These photons 214 pass through the ARC 209, which is designed to minimize the number of photons 214 that are reflected away from the IBC solar cell 205. The ARC 209 may be comprised of a SiNx layer in one instance. The photons 214 enter through the n+ front surface field 207. The photons 214 with sufficient energy (above the bandgap of the semiconductor) are able to promote an electron within the valence band of the semiconductor material to the conduction band. Associated with this free electron is a corresponding positively charged hole in the valence band.
On the back side of the IBC solar cell 205 is an emitter region 215. The doping pattern of the emitter region 215 is alternating p-type and n-type dopant regions in this particular embodiment. The n+ back surface field 204 may be approximately 450 μm in width and doped with phosphorus or other n-type dopants. The p+ emitter 203 may be approximately 1450 μm in width and doped with boron or other p-type dopants. This doping may enable the junction in the IBC solar cell 205 to function or have increased efficiency. This IBC solar cell 205 also includes a passivating layer 212, n-type contact fingers 210, p-type contact fingers 211, and contact holes 213 through the passivating layer 212.
To form the IBC solar cell, at least two patterned doping steps may be required. These patterned doping steps need to be aligned to prevent the p+ emitter 203 and the n+ back surface field 204 from overlapping. In one instance, the alignment needs to be between approximately 5-50 μm. Poor alignment or overlapping may be prevented by leaving a gap between the p+ emitter 203 and the n+ back surface field 204, but this may degrade performance of the IBC solar cell depending on the size of the gap. Even when properly aligned, such patterned doping may have large manufacturing costs. For example, photolithography or hard masks (such as an oxide) may be used, but both are expensive and require extra process steps. Furthermore, it may be difficult to construct a shadow mask with long, thin fingers that may be used, for example, with certain implant steps. Therefore, there is a need in the art for an improved method of doping solar cells and, more particularly, an improved method of doping IBC solar cells using ion implantation.